The production of megakaryocytes from progenitor cells in the marrow can be stimulated by a lineage-specific factor - megakaryocyte colony-stimulating factor (Meg-CSF). The lineage-specific stimulation of megakaryocytes by Meg-CSF and subsequent increase in platelet number has great clinical significance in treating thrombocytopenia resulting from anti-cancer or anti-HIV therapies. The production of polyclonal and monoclonal antibodies to Meg-CSF proposed by this Phase I SBIR would facilitate the purification of Meg-CSF by affinity chromatography, and the identification of recombinant clones expressing Meg-CSF cDNA. In addition, such antibodies will enable the development of assays, such as an enzyme-linked immunosorbent assay (ELISA), to determine serum or urine Meg-CSF levels in those patients undergoing chemotherapy or who have thrombocytopenia from other causes. Rabbits will be immunized to obtain polyclonal antibodies and both in vitro and in vivo immunization of mice is proposed for monoclonal antibody production.